Many designs for HPS arc lamps are currently known in the art. These lamps typically have a polycrystalline alumina (PCA) arc tube filled with a mixture of gases, including xenon, and one or more amalgams of sodium and mercury which form an arc discharge.
The sodium and mercury components of the fill material are primarily responsible for the light output characteristics of the lamp. For example, the ratio of the mixture affects the color spectrum of the light output. The xenon component of the gas mixture primarily helps to improve lamp life and efficiency. However, HPS lamps including xenon at a relatively high pressure in the gas mixture are difficult to start reliably without an external starting aid.
External starting aids generally take the form of an elongated conductive element, such as a metal ignition wire, or a coiled ignition filament. The starting aid is positioned in contact with an outer surface of the PCA arc tube. The starting aid is connected to one electrical power lead of the lamp. When an arc is formed and the lamp begins to warm up, either power is removed from the starting aid, or the starting aid is moved away from the arc tube, so as to prevent electric field accelerated sodium diffusion through the arc tube wall. Such sodium diffusion would adversely affect lamp life.
In one prior art design, when the temperature of the lamp rises to a certain value, the starting aid is disconnected from the electrical power lead, for example by means of a bi-metallic switch electrically connected between the starting aid and the electrical power lead.
Another HPS lamp of the prior art includes a metal frame, from which the starting aid is stretched across a surface of the arc tube. The metal frame is connected to one power lead of the lamp. Bi-metallic strips attached to the frame lift the starting aid from the surface of the arc tube when the lamp approaches operating temperature.
There are a number of problems inherent in prior art starting aid designs. The ignition wire or coiled ignition filament of the prior art is suspended from the lamp frame. This involves a difficult and costly welding operation. Furthermore, in the absence of direct attachment of the ignition wire or coiled ignition filament to the arc tube, the wire or filament may sag away from the arc tube due to the high temperature of operation of these lamps. Additionally, the bi-metallic switches of the prior art are typically attached to the lamp frame. Thus, they are heated by radiation, rather than by conduction, which makes the performance of a switch design vary when it is used in lamps of different wattage. Finally, the prior art attachment of starting aids to the lamp frame and the prior art bi-metallic switch designs result in a complex and costly assembly process.